Field of the Invention
This invention relates to a process for preparing a blue emitting phosphor, and more particularly to a process for preparing an alkaline earth metal aluminate phosphor activated with divalent europium.
This invention is particularly concerned with a process for preparing a phosphor sealed in an air-tight container of a gaseous discharge light emitting element which effects a high radiation efficiency and luminance. In the light emitting element, the discharge gap is in the range of 0.1 to 3.0 mm and the pressure of the gas sealed in the container is such that the product of the pressure and the discharge gap (hereinafter referred to as "pd product") is in the range of 30 to 300 Torr.multidot.mm.
Description of the Prior Art
It has known in the art to excite a phosphor with ultraviolet rays emitted by gaseous discharge to cause the phosphor to emit light. For instance, fluorescent lamps employ a phosphor which is excited by ultraviolet rays having a wavelength of 253.7 nm emitted by a discharge in a mercury vapor. When the gap between a pair of discharge electrodes within a small lamp is smaller than 3 mm, the pressure of the gas sealed in the lamp is required to be as high as several tens to several hundreds of Torrs in accordance with Paschen's law. In small lamps or image display panels in which the gap between the electrodes is smaller than 3 mm, therefore, the radiation of ultraviolet rays cannot be obtained effectively since the vapor pressure of mercury sealed within the lamp together with argon is as low as 10.sup.-3 Torr or less at 15.degree. C. and 10.sup.-2 Torr or less even at 40.degree. C. Therefore, in general, this kind of discharge lamp and other similar devices usually employ a rare gas, a hydrogen gas or a nitrogen gas or an appropriate mixture of these gases because with such gases a pressure of several tens to several hundreds of Torrs can easily be obtained at room temperature. The ultraviolet rays emitted by the gaseous discharge in the above mentioned gas or gas mixture have radiation spectra of high intensity mostly within the so-called vacuum ultraviolet region corresponding to the wavelength of shorter than 200 nm.
As a conventional phosphor which is used in image display panels employing gaseous discharge with a discharge gap of 0.1 to 3.0 mm and pd product of 30 to 300 Torr.multidot.mm and emitting blue light under excitation by ultraviolet rays, cathode rays or X-rays, there have been well known divalent europium activated alkaline earth metal aluminate phosphors (BaO.6Al.sub.2 O.sub.3 :Eu.sup.2+, MgO.BaO.8Al.sub.2 O.sub.3 :Eu.sup.2+, 2MgO.BaO.7Al.sub.2 O.sub.3 :Eu.sup.2+ etc.) as disclosed in Japanese Patent Public Disclosure No. 77893/1974. These phosphors, however, are known to emit blue light with high emission performance only under excitation by ultraviolet rays emitted by discharge in mercury vapor, that is ultraviolet rays having the wavelength of 253.7 nm (in case of low pressure mercury lamp) or 365 nm (in case of high pressure mercury lamp). However, the emission performance of the phosphor under excitation by vacuum ultraviolet rays having the wavelength shorter than 200 nm is hardly known.